I'll start by stating, my background is electrical
engineering. I also served in the Air Force and believe in over-kill rather than be caught short.
I just finished designing and installing a solar
system in my pickup camper. My goal was to generate enough power for full-time winter use without using tilting panels. I feel it will work for several days with overcast conditions. In winter I would be fighting low angles of solar
incidence, limited sunshine, and higher energy demands. I considered tilting panels however many of these mechanisms cost nearly as much as buying additional panels. The projected increase with tilt was 30%. Adding two more panels would increase my energy input 50%...a better deal with the benefit of more power all the time. Tilting panels are a nuisance. They run the risk of forgetting to lower them and damaging from tree limbs.
I plan to boondock almost exclusively. So, reliable power is important to me. I will be living in my camper extensively during the next year as I complete a yacht restoration project. This extra power provides me all the comforts I need.
Physical Constraints: My battery
compartment holds one 165 lb 225 A-H 8D gel cell battery
. I have space in the generator
compartment for one or two more if weight
was not a factor. One is fine. Perhaps in the winter I might need another battery
if I were in the cold north. My plan is to use the generator
space to store an outdoor propane
Battery: Gel cells are noted for their cycling longevity. If discharged to 70%, they can go 3000 cycles. That is over 8 years with daily cycling. At a 50% discharge, longevity drops to 1800-2000 cycles--5 years. I got this battery for $50 as a take out from a yacht and it had not seen much use. My plan is to use it up until it fails and then consider replacement options--probably replacing it with the same although I hope lithium techno guy prices drop in that time period. I am hoping for 3 years use with gentle loving care. It is a 2011 battery.
Panels: I bought my four 105 watt mono crystalline solar panels (420 watts total) at Home Depot and used my 10% military discount. Cost was $132 per panel. The rule of thumb on yachts is to figure 5 hours at full power for a conservative daily power input. From experience with fixed panels this is a good estimator. For 5 hours that works out to 2100 watts or 175 A-H.
Optimization: Solar panels stop charging when the voltage drops below 12V. I arranged my four panels in an unobstructed area on my roof. I have two panels in series 36V nominal voltage, in parallel with another pair at the same voltage. This means my panels start charging earlier and keep charging until night fall
as the voltage is typically well above 12V most of the time. If shaded this configuration would produce less power that four panels in parallel. I could have put the all in series for 72 Volts but the shading factor would be worse.
Charge Controller: I bought a Tracer 4210 40 Amp MPPT charge controller--a MOPT is a more efficient type of controller that conveys excess voltage into higher current levels at 12V and can be biased for three stage charging of gel cell batteries. So the maximum amount of energy is going into my battery. I am not sure I like this charge controller. I would considers different brand next time as the charging voltages seem a big high on float--just under 14 volts.
Shore Power Charging: I upgraded my converter to a modern multistage charger, mostly to condition the battery. I used it initially and now no longer use it. It was difficult getting the necessary information from the manufacturer but I did find one that had a jumper to drop the charging bias voltage by 0.4 Volts to meet the lower voltage requirements of a gel cell battery. My model, made by progressive dynamics is a 35 amp model. Note, they offer 35 Amp, 45 Amp, and 55 Amp chargers. Gel Cell batteries need slower charging so I chose the smallest model converter upgrade to save my gel cell from damage due to overcharging.
Inverter: I installed a Sunforce 11240 1000 watt pure sine wave inverter. I do not recommend it unless you are willing to modify it. I wired it such that it powers only the camper outlets, which are also powered through the shore power switch when that source is selected . I made it idiot proof because it was the smart thing to do. The AC outlet circuits energized will include TV, TiVo, Blu-ray player, Mac Mini, Apple Airport Extreme WiFi, Netgear switch, and a few other loads. My 1250 watt microwave
requires more power than my inverter will deliver. If I can bond the inverter outputs together, that supply 500 Watts each, I can buy a smaller 1000 watt microwave
that could be energized by the inverter. Larger inverters are heavier and draw more power. I wanted to size my inverter to match the loads.
I also bought a cheap model inverter, with plastic sides, thumb screw DC terminals and no way to permanently mount it. I modified it by removing the fake plastic heat sinks on the sides and replaced these with sheet metals sides with mounting flanges. I replaced the thumb screws with 8mmx125 nuts. That's how I made a $180 Chinese inverter work for me. If it cramps out I'll get a bigger one and upgrading its associated DC wire. I may pull it out and open it up to bond the two 500 watt outputs into one--of that is possible so that I can run a microwave
Results: Since installing the solar panels I'm finding the big surplus of power is working well. I have seen 1-2 amps of charging when heavily obscured by clouds. This is plenty when I don't use the furnace
. I used 50 A-H's last night running the furnace
about 30% duty cycle (low was 20 degrees F rising to 50 degrees F now), which draws 3.6 Amps, the inverter, and parasitic loads.
My only regret is that I didn't choose a grid-tie inverter that could be hooked up at home to reduce energy costs. These are much more expensive. With a grid tie I could use my excess power to offset my electric bill.
Connectors: I used MC4 solar power
connectors (highly recommended). I used heavily insulated 10 gauge conductors, and 5/16" marine cable glands to bring the wire into the camper. I also used MC4 junction terminators to join the wires on the roof, securing these to the solar panels with extra long 3/16 inch pop rivets. I highly recommend these solar power
connectors; they work great, are easily serviced or expanded. If I choose to add more panels it will be very easy to do.
A few numbers: At 9:30 this morning here at latitude 41 N. I was putting 10 amps into my battery which were at about 76%. At noon here now it is putting in over 16.7 Amps. I am down 25.2 A-H on a total capacity of 225 A-H. 87% charge here at noon. At 12:30 I am at 90% and current varies with the cloud
cover. 92.8% at 1 PM. It shows fully charged at 2:40, but I'm sure it is still a few percent below fully charged, however for practical purposes, it works great. I have high confidence it will work in more cloudy conditions as long as my used battery holds out.
So what does it all mean?
Excess solar capacity is always good.
Excess battery capacity means your % discharge is less and battery life goes way up.
The penalty for more battery weight
is not worth it, as it is easy to replace a battery that to carry it around for long periods. I could add another battery but do not need it unless I was seeing more than three days of overcast conditions in wintertime. A second battery would give me 5 days reserve at 50% discharge--that is huge. I could then go a week at 75% discharge and two sunny days would top me off again.
I would choose greater battery capacity for difficult winter conditions--far northern Canada in the winter, if my loads were higher, for mission critical applications, or for fixed location use.
Could I have gotten by with fewer solar panels? Yes, for not much cost savings at the expense of worry about my system.
Charging voltages parameters vary by the manufacturer. Gel cells are fussy on these, but best suited to solar charging in my opinion. I decided not to worry about by battery too much. Time will tell on this. AGM or flooded batteries are not so worrisome but don't last as long. For shore charger only use, without solar panels, I would choose AGM and a bigger, faster charger, or flooded batteries if I was willing to deal with the maintenance.